1. Field of the Invention
The present invention relates generally to bone conduction devices, and more particularly, to fitting a bone conduction device.
2. Related Art
Hearing loss, which may be due to many different causes, is generally of two types, conductive or sensorineural. In many people who are profoundly deaf, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or destruction of the hair cells in the cochlea which transduce acoustic signals into nerve impulses. Various prosthetic hearing implants have been developed to provide individuals who suffer from sensorineural hearing loss with the ability to perceive sound. One such prosthetic hearing implant is referred to as a cochlear implant. Cochlear implants use an electrode array implanted in the cochlea of a recipient to provide electrical stimulation directly to the cochlea nerve, thereby causing a hearing sensation.
Conductive hearing loss occurs when the normal mechanical pathways that provide sound to hair cells in the cochlea are impeded, for example, by damage to the ossicular chain or ear canal. Individuals who suffer from conductive hearing loss may still have some form of residual hearing because the hair cells in the cochlea are generally undamaged.
Individuals who suffer from conductive hearing loss are typically not considered to be candidates for a cochlear implant due to the irreversible nature of the cochlear implant. Specifically, insertion of the electrode array into a recipient's cochlea results in the destruction of a majority of hair cells within the cochlea. This results in the loss of residual hearing by the recipient.
Rather, individuals suffering from conductive hearing loss typically receive an acoustic hearing aid. Hearing aids rely on principles of air conduction to transmit acoustic signals through the outer and middle ears to the cochlea. In particular, a hearing aid is typically positioned in the recipient's ear canal to amplify sound received at the outer ear of the recipient. This amplified sound reaches the cochlea and causes motion of the cochlear fluid and stimulation of the cochlea hair cells.
Unfortunately, not all individuals who suffer from conductive hearing loss are able to derive suitable benefit from hearing aids. For example, some individuals are prone to chronic inflammation or infection of the ear canal and cannot wear hearing aids. Other individuals have malformed or absent outer ear and/or ear canals as a result of a birth defect, or as a result of common medical conditions such as Treacher Collins syndrome or Microtia. Furthermore, hearing aids are typically unsuitable for individuals who suffer from single-sided deafness (total hearing loss only in one ear) or individuals who suffer from mixed hearing losses (i.e., combinations of sensorineural and conductive hearing loss).
When an individual having fully functioning hearing receives a sound, the sound is transmitted to the cochlea via two primary mechanisms: air conduction and bone conduction. As noted above, hearing aids rely primarily on the principles of air conduction. In contrast, other devices, referred to as bone conduction devices, rely predominantly on vibration of the bones of the recipients skull to provide acoustic signals to the cochlea.
Those individuals who cannot derive suitable benefit from hearing aids may benefit from bone conduction devices. Bone conduction devices convert a received sound into a mechanical vibration representative of the received sound. This vibration is then transferred to the bone structure of the skull, causing vibration of the recipient's skull. This skull vibration results in motion of the cochlear fluid. Hair cells inside the cochlea are responsive to this motion of the cochlea fluid, thereby generating nerve impulses, which result in the perception of the received sound.